As is well known, flow meters are used to perform flow rate measurements. For example, fuel dispensers in retail service station environments include flow meters that measure volumetric flow rate of the fuel as it is dispensed. Such flow meters are typically required to comply with weights and measures regulatory requirements that mandate a high level of accuracy. This ensures that the customer is neither overcharged nor undercharged for the fuel purchase. Typically, either positive displacement meters or inferential meters have been used for this purpose.
Modern service station fuel dispensers monitor the amount of fuel delivered to a customer's vehicle by counting the number of pulses generated by a pulser. Pulsers are electromechanical devices operatively connected to the flow meter which generate a pulse train as the fuel is dispensed. Each pulse represents a known volume of fuel passing through the meter. Attempts have been made to interfere with or alter the signal emitted by the pulser in order to modify the calculated volume of fuel dispensed. For example, a dishonest consumer may desire to report a lower fuel volume in order to steal a portion of the fuel.
Many older pulsers use general purpose input/output (I/O) for communication. However, serial pulsers are also known. Serial pulsers are connected to a serial communications port on a controller board or the like of the dispenser. Such pulsers typically have a processor that communicates pulse data to the dispenser controller via a protocol.
For a variety of reasons, fuel volume or flow rate measurement technologies have limited accuracy. For example, various mechanical components of a flow meter may undergo wear during the life of the meter. This wear, also known as meter drift, introduces an error from a meter's initial calibration state. Typically, flow meters also have limited accuracy across a finite range of flow rates, such as at low flow rates. Therefore, flow meters often require periodic manual or electronic calibration. Also, some flow meters are electronically self-calibrating based on the amount of fuel dispensed over the service life of the meter.
As an example of electronic calibration, an authorized technician may toggle a switch located in the dispenser electronics compartment to place a fuel dispenser in a calibration mode. The technician then actuates the fuel dispenser and dispenses a quantity of fuel into a metered vessel. Next, the technician uses a keypad or the like to input the measured volume of dispensed fuel, and the keypad transmits that value to a dispenser controller. The controller includes electronics that communicate with the pulser to obtain the raw pulse data and calculate the purported volume of fuel dispensed. The controller then executes software that, based upon data received from the pulser and the measured volume entered on the keypad, calculates a calibration factor and stores that calibration factor in memory.
The calibration procedure may be performed over a range of flow rates to arrive at a calibration curve. Often, the dispenser controller will employ the meter calibration factor or calibration curve to alter or correct the pulse data obtained from a specific meter. Alternatively, or in addition, the volumetric data calculated from the raw pulse data may be revised to obtain the correct volume of fuel dispensed. Modern fuel dispensers may store both the raw pulse data and the total corrected and uncorrected volume data over the life of the pulser in one or more circuit boards in the dispenser electronics compartment.
Additionally, the volume of liquid fuel is somewhat dependent on temperature (i.e., it expands when heated and contracts when cooled). Various governmental bodies have from time to time required temperature effects to be taken into account. Prior art solutions provide temperature compensation by sending signals from thermometric probes located in a flow meter to a first circuit in the dispenser's lower fuel handling compartment, to a second circuit in the dispenser's upper electronics compartment via an intrinsically safe connection, and finally to a computation device designed to combine the temperature data and pulser data. The computation device employs a volume correction factor (VCF) to compensate the pulser data so as to account for temperature variations. Detailed information regarding temperature compensation of dispensed fuel is disclosed in U.S. Pat. No. 5,557,084 to Myers et al., entitled “Temperature Compensating Fuel Dispenser,” the entire disclosure of which is incorporated herein by reference for all purposes.